Subject: Re: [PATCH v7 3/8] powerpc: detect the trusted boot state of the system

From: Michael Ellerman <[email-masked]>
Date: Tue, 15 Oct 2019 21:23:19 +1100

Nayna Jain <[email-masked]> writes:
> PowerNV systems enables the IMA measurement rules only if the
> trusted boot is enabled on the system.

That confused me a lot. But the key is the distinction between appraisal
rules vs measurement rules, right?

I think it would be clearer if it was phrased as a positive statement, eg:

  On PowerNV systems when trusted boot is enabled, additional IMA rules
  are enabled to implement measurement.

Or something like that.

> This patch adds the function to detect if the system has trusted
> boot enabled.

It would probably help people to briefly explain the difference between
secure vs trusted boot.

> diff --git a/arch/powerpc/include/asm/secure_boot.h b/arch/powerpc/include/asm/secure_boot.h
> index 23d2ef2f1f7b..ecd08515e301 100644
> --- a/arch/powerpc/include/asm/secure_boot.h
> +++ b/arch/powerpc/include/asm/secure_boot.h
> @@ -12,6 +12,7 @@
>  
>  bool is_powerpc_os_secureboot_enabled(void);
>  struct device_node *get_powerpc_os_sb_node(void);
> +bool is_powerpc_trustedboot_enabled(void);
>  
>  #else
>  
> @@ -25,5 +26,10 @@ static inline struct device_node *get_powerpc_os_sb_node(void)
>  	return NULL;
>  }
>  
> +static inline bool is_powerpc_os_trustedboot_enabled(void)

That has an extra "_os" in it.

> +{
> +	return false;
> +}
> +
>  #endif
>  #endif
> diff --git a/arch/powerpc/kernel/secure_boot.c b/arch/powerpc/kernel/secure_boot.c
> index 0488dbcab6b9..9d5ac1b39e46 100644
> --- a/arch/powerpc/kernel/secure_boot.c
> +++ b/arch/powerpc/kernel/secure_boot.c
> @@ -7,6 +7,27 @@
>  #include <linux/of.h>
>  #include <asm/secure_boot.h>
>  
> +static const char * const fwsecureboot_compat[] = {
> +	"ibm,secureboot-v1",
> +	"ibm,secureboot-v2",
> +	NULL,
> +};
> +
> +static struct device_node *get_powerpc_fw_sb_node(void)
> +{
> +	struct device_node *node;
> +	int i;
> +
> +	for (i = 0; i < ARRAY_SIZE(fwsecureboot_compat); ++i) {
> +		node = of_find_compatible_node(NULL, NULL,
> +					       fwsecureboot_compat[i]);
> +		if (node)
> +			return node;
> +	}
> +
> +	return NULL;
> +}

You shouldn't need to do that by hand, instead use
of_find_matching_node(), eg:

static struct device_node *get_powerpc_fw_sb_node(void)
{
	static const struct of_device_id ids[] = {
		{ .compatible = "ibm,secureboot-v1", },
		{ .compatible = "ibm,secureboot-v2", },
		{},
	};

	return of_find_matching_node(NULL, ids);
}

> @@ -40,3 +61,17 @@ bool is_powerpc_os_secureboot_enabled(void)
>  	pr_info("secureboot mode disabled\n");
>  	return false;
>  }
> +
> +bool is_powerpc_trustedboot_enabled(void)
> +{
> +	struct device_node *node;
> +
> +	node = get_powerpc_fw_sb_node();
> +	if (node && (of_find_property(node, "trusted-enabled", NULL))) {

Again this can use of_property_read_bool(), which copes with a NULL node
also, so just:

+	if (of_property_read_bool(node, "trusted-enabled"))) {

> +		pr_info("trustedboot mode enabled\n");
> +		return true;
> +	}
> +
> +	pr_info("trustedboot mode disabled\n");
> +	return false;
> +}
> -- 
> 2.20.1

cheers